在Array擴展Swift上不能調用自己的函數

我在swift中擴展Array類。我需要排序的數組，但是當我嘗試把它給我的錯誤Immutable value of type 'Array<T>' only has mutating members named 'sort'在Array擴展Swift上不能調用自己的函數

這裏是我的分機：

import UIKit 

extension Array { 

    func sortedArrayByTimeUsingKey(key: String, ascending: Bool) { 

     sort { (firstObject, secondObject) -> Bool in 

      let firstDict = firstObject as [String: AnyObject] 
      let secondDict = secondObject as [String: AnyObject] 

      let firstDateString = firstDict[key]! as String 
      let secondDateString = secondDict[key]! as String 

      let dateFormatter = DateFormatter() 

      let firstDate = dateFormatter.dateFromString(firstDateString)! 
      let secondDate = dateFormatter.dateFromString(secondDateString)! 

      let order = firstDate.compare(secondDate) 

      switch order { 
      case .OrderedAscending: 
       return ascending ? true : false 
      case .OrderedDescending: 
       return ascending ? true : false 
      default: 
       return true 
      } 
     } 
    } 

}

我怎麼能投self是可變的或者我怎麼能檢查是否self是可變的？

來源

2014-09-27 jamespick